The present invention relates to a vacuum power interrupting device, and more particularly but not exclusively to a vacuum power interrupting device adapted to be mounted on a power board or a swtiching board.
In U.S. Pat. No. 4,365,127, there has been proposed a vacuum power interrupter comprising a vacuum vessel consisting of a bell shaped metallic casing, and an insulating end plate of ceramic hermetically brazed to the opening end of the metallic casing, and a pair of contact rods disposed within the vacuum vessel so that one is in contact with the other or away therefrom.
This type of the vacuum power interrupter makes it easy to enlarge an outer radius of the vacuum vessel for an interrupter capable of interrupting a large electric current, as compared with a second type of the vacuum power interrupter comprising a cylindrical insulating envelope, metallic end plates hermetically brazed to the axial ends of the insulating envelope, respectively, and stationary and movable contact rods disposed within the insulating envelope so that the latter is movable relative to the former.
The first type of the vacuum power interrupter further is easy to fabricate and is inexpensive because of the fact that the insulating end plate of ceramic is used instead of an insulating envelope of ceramic which is costly.
However, with the first type of vacuum power interrupter, the following drawback is pointed out:
The area of the insulating end plate is small as compared with that of the insulating envelope. As a result, the flash-over path through an air path or tracking path over a solid surface of the insulating end plate is small. Accordingly, the insulating withstanding voltage becomes small, resulting in that the insulating withstanding voltage is smaller than that of the second type of the vacuum power interrupter. As a result, the first type of the vacuum power interrupter makes it difficult to interrupt a high voltage.